Speaking on behalf of Brunel University London's Britain in Europe think-tank, Sir Geoffrey said the UK should "stick with our neighbours", particularly over issues such as immigration which rely on close cooperation, in an article published in the Solicitors Journal today.
"We should not break up the club and return to being a nation state in a world largely centred elsewhere. We should stick with our neighbours. Immigration, is, of course, the ultimate neighbour issue. It is in our own best interest to treat your neighbour as yourself," added Sir Geoffrey, the leading lawyer in prosecuting former president of Serbia Slobodan Milosevic for genocide and crimes against humanity.
"Those of us who recall the days of anti-immigration and racial discrimination will remember the embarrassment that slowly dawned, forced on us by writers and filmmakers. We can recall how embarrassment turned to a sense of disgrace.
"On immigration, we must act now jointly with Europeans within Europe as good neighbours in order that we may all ultimately act together in a dangerous, troubled world."
Brunel's Britain in Europe think-tank was established to focus on human rights and criminal justice in the European Union.
BiE's director said Dr Giannoulopoulos said: "Sir Geoffrey's opinion on the referendum is shaped by decades of personal experience of the international legal order.
"As a key player in international criminal tribunals and other international organisations, he rightly cautions on how the UK should proceed from here so as not to lose the ability to tackle global problems, in collaboration with others and, notably, its European neighbours."